Conformational changes during amyloid fibril formation of pancreatic thiol proteinase inhibitor: effect of copper and zinc

Heparin promotes rapid fibrillation of the basic Parathyroid Hormone at physiological pH

In acidic secretory granules of mammalian cells, peptide hormones including the parathyroid hormone (PTH) are presumably stored in the form of functional amyloid fibrils. Mature PTH, however, is considerably positively charged in acidic environments, a condition known to impede unassisted self-aggregation into fibrils. Here, we studied the role of the polyanion heparin on promoting fibril formation of PTH. Employing ITC, CD spectroscopy, NMR, SAXS and fluorescence-based assays we could demonstrate that heparin binds PTH with submicromolar affinity and facilitates its conversion into fibrillar seeds, enabling rapid formation of amyloid fibrils under acidic conditions. In absence of heparin, PTH remained in a soluble monomeric state. We suspect that heparin-like surfaces are required in vivo to convert PTH efficiently into fibrillar deposits.

A biophysical insight into the formation of aggregates upon trifluoroethanol induced structural and conformational changes in garlic cystatin

Intrinsic and extrinsic factors are responsible for the transition of soluble proteins into aggregated form. Trifluoroethanol is among such potent extrinsic factor which facilitates the formation of aggregated structure. It disrupts the interactive forces and destabilizes the native structure of the protein. The present study investigates the effect of trifluoroethanol (TFE) on garlic cystatin. Garlic cystatin was incubated with increasing concentration of TFE (0-90% v/v) for 4 h. Incubation of GPC with TFE induces structural changes thereby resulting in the formation of aggregates. Inactivation of garlic phytocystatin was confirmed by cysteine proteinase inhibitory activity. Garlic cystatin at 30% TFE exhibits native-like secondary structure and high ANS fluorescence, thus suggesting the presence of molten globule state. Circular dichroism and FTIR confirmed the transition of the native alpha-helical structure of garlic cystatin to the beta-sheet structure at 60% TFE. Furthermore, increased ThT fluorescence and redshift in Congo red absorbance assay confirmed the presence of aggregates. Rayleigh and turbidity assay was also performed to validate the aggregation results. Scanning electron microscopy was followed to analyze the morphological changes which confirm the presence of sheath-like structure at 60% TFE. The study sheds light on the conformational behavior of a plant protein when kept under stress condition induced by an extrinsic factor.

Methylglyoxal induced glycation and aggregation of human serum albumin: Biochemical and biophysical approach

Serum protein glycation and formation of advanced glycation end products (AGEs) correlates with many diseases viz. diabetes signifying the importance of studying the glycation pattern of serum proteins. In our present study, methylglyoxal was investigated for its effect on the structure of human serum albumin (HSA); exploring the formation of AGEs and aggregates of HSA. The analytical tools employed includes intrinsic and extrinsic fluorescence, UV spectroscopy, far UV circular dichroism, Thioflavin T fluorescence, congo red binding, polyacrylamide gel electrophoresis (PAGE). UV and fluorescence spectroscopy revealed the structural transition of native HSA evident by new peaks and increased absorbance in UV spectra and quenched fluorescence in the presence of MG. Far UV CD spectroscopy revealed MG induced secondary structural alteration evident by reduced α-helical content. AGEs formation was confirmed by AGEs specific fluorescence. Increased ThT fluorescence and CR absorbance of 10mM MG incubated HSA suggests that glycated HSA results in the formation of aggregates of HSA. SEM and TEM were reported to have an insight of these aggregates. Molecular docking was also utilized to see site specific interaction of MG-HSA. This study is clinically significant as HSA is a clinically relevant protein which plays a crucial role in many diseases.

pH induced single step shift of hydrophobic patches followed by formation of an MG state and an amyloidogenic intermediate in Lima Bean Trypsin Inhibitor (LBTI)

Lima Bean Trypsin Inhibitor (LBTI) is 83 residues monomeric protein of 9.0 KDa, consisting of six antiparallel β-strands and can undergo concentration dependant dimerization. We have tried to characterize folding intermediates of LBTI under equilibrium denaturation conditions. We have used various spectroscopic and microscopic techniques to understand the folding and misfolding pathways. LBTI forms molten globule structure at pH 2 and amyloidiogenic intermediate state (I^a) at pH 4. pH induced Shifting of surface exposed hydrophobic patches and that followed by withdrawal of the lone tyrosine residue (Y69) towards nonpolar environment have been reported. Denaturation profile of native and molten globule (MG) states of LBTI in presence of guanidine hydrochloride show sigmoidal curves with non-coincidental and irreversible behaviour in both states. Concentration dependent amyloid fibril formation was confirmed by Thioflavin T and Congo Red binding and its morphology was studied by transmission electron microscopy (TEM). This is the first report on biophysical characterization of folding intermediates of LBTI and its aggregation behaviour to the best of our knowledge.

Physico-chemical stress induced amyloid formation in insulin: Amyloid characterization, cytotoxicity analysis against human neuroblastoma cell lines and its prevention using black seeds (Nigella sativa)

OBJECTIVE

To investigate the aggregation and fibrillation of insulin at low pH and moderate temperature; and to further test the aggregated insulin for its cytotoxicity on human neuroblastoma (SH-SY5Y) cell line and inhibition of the cytotoxicity by black seeds (Nigella sativa) extract.

METHODS

Bovine pancreatic insulin was incubated at pH 2.0, 45 ℃ under stirring condition at 400 r/min for 24 h. Amyloids like structures in the aggregated insulin were characterized using various techniques such as thioflavin T assay (ThT), 1-anilinonaphthalene-8-sulfonic acid (ANS) fluorescence, circular dichroism (CD) and dynamic light scattering (DLS). Moreover, cytotoxicity of aggregated insulin was monitored on SH-SY5Y cell line in the presence and absence of black seeds extract using standard 3-(4,5-dimethylthiazol-2-Yl)-2,5-diphenyltetrazolium bromide (MTT), lactate dehydrogenase (LDH) and reactive oxygen species (ROS) assay kit.

RESULTS

Our finding demonstrated that insulin under the mentioned conditions formed amyloid-like structure. ANS binding to aggregated insulin showed increase in fluorescence, suggesting structural change and increase in hydrophobicity in insulin occurring during the fibril formation. DLS measurement revealed progressive increase in hydrodynamic radius of aggregated insulin. Cytotoxicity assays illustrated aggregated insulin induced apoptosis in SH-SY5Y cell through ROS formation. Moreover, LDH measurement showed aggregated insulin triggered membrane damage in SH-SY5Y cell lines. Black seeds extract was found to inhibit amyloid formation and protected the cells against amyloid toxicity.

CONCLUSION

Insulin molded into amyloid like structure at low pH and under stirring conditions. Characterization of insulin aggregates illustrated conformational change in insulin and it experiences α-helix to β-sheet transition during the course of fibrillation. Black seeds extract inhibited amyloid progression of insulin via ROS scavenging and restrained the cytotoxicity caused by insulin fibrils suggesting black seeds containing polyphenols may serve as a lead structure to a novel anti-amyloidogenic drugs.

Conformational behaviour and aggregation of chickpea cystatin in trifluoroethanol: effects of epicatechin and tannic acid

Conformational alterations and aggregates of chickpea cystatin (CPC) were investigated upon sequential addition of trifluoroethanol (TFE) over a range of 0-70% v/v. CPC on 30% and 40% v/v TFE addition exhibited non-native β-sheet, altered intrinsic fluorescence, increased thioflavin T fluorescence, prominent red shifted shoulder peak in Congo red absorbance, and enhanced turbidity as well as Rayleigh scattering, suggesting the aggregate formation. TEM results confirmed the formation of fibrillar aggregates at 30% and 40% v/v TFE. On increasing concentration of TFE to 70% v/v, CPC showed retention of native-like secondary structure, increased intrinsic and ANS fluorescence. Thus our results show that favourable condition for fibrillation of CPC is in the range of 30-40% TFE. Moreover, anti-aggregational effects of polyphenols, epicatechin (EC) and tannic acid (TA) were analysed using ThT binding assay and other biophysical assays. EC and TA produced a concentration dependent decline in ThT fluorescence suggesting inhibition of the fibril formation. Furthermore, TA in comparison to EC, served as a more effective inhibitor against amyloid fibril formation of CPC. This work supports the universality of the amyloid-like aggregation not restricted to some special categories of protein and the fact that this aggregation can be prevented.